A. Field of the Invention
This invention relates to the design of cutters used in fixed cutter drill bits such as are used for drilling holes for blasting, and oil and gas exploration and production. In particular, this invention relates to cutters for use on rotary drag bits which are configured to maximize wear resistance and to enhance drill bit performance.
B. The Background Art
It is known in the prior art to construct drill bits for drilling holes in rock formations by affixing a plurality of discrete cutting elements made of a superhard material (typically diamond) to a substrate of some other material, such as tungsten carbide. In the past, chips of diamond set in the surface of a drill bit, as disclosed by Havlick (U.S. Pat. No. 2,264,440) have been used. More recently it has become common for drill bits to include cutting elements which are composites of a substrate material (e.g. tungsten carbide) and a superhard material (e.g. polycrystalline diamond). The superhard material most often serves as a surface material, but may also be used in internal reinforcing structures. These composite cutting elements are usually in the form of either short, cylindrical "compacts" which are used primarily in rotary drag bits, or buttons or inserts which are used in rolling cone or percussion bits.
The simplest form of compact is simply a short cylinder (typically with a diameter greater than its height) of substrate material with a uniform layer of superhard material on one face. This type of compact is described in the patents of Daniels (U.S. Pat. No. 4,156,329) and Bovenkerk (U.S. Pat. No. 4,268,276). The superhard material provides a wear resistant cutting edge. Buttons and inserts may also be constructed with a superhard surface over a substrate material (Waldenstrom, U.S. Pat. No. 5,335,738 and Keshavan, U.S. Pat. No. 5,158,148).
Prior art improvements to the basic compact design include modifications to the interface between the substrate and the superhard material. Many previous patents describe modifications to the interface geometry which improve the transfer of stresses between the different materials, e.g,. patterns of linear ridges as disclosed by Dennis (U.S. Pat. Nos. 4,592,433 and 5,120,327), Aronssen (U.S. Pat. No. 4,764,434) and Hall (U.S. Pat. No. 4,629,373), or ridges extending radially outward (Flood, U.S. Pat. No. 5,486,137; Smith, U.S. Pat. No. 5,351,772; and Dennis, U.S. Pat. Nos. 5,379,854 and 5,544,713). Hardy et al (U.S. Pat. No. 5,355,969) describe a cutter design having a concentric pattern of ridges at the interface. Matthias et al. (U.K Patent No. 2,290,328) disclose cutters having various patterns of ridges at the interface in the region of the cutting edge. Matthias (U.K. 2,290,327) discloses a cutter with a star-shaped pattern of ridges which extend into the substrate.
Projections which extend from the substrate into the superhard surface (Waldenstrom, U.S. Pat. Nos. 5,217,081 and 5,335,738), Frushour (U.S. Pat. No. 5,564,511), Hardy (U.S. Pat. No. 5,355,969) or from the surface into the substrate (Griffin, U.K. Patent No. 2,290,326) have also been disclosed. These projections are generally rounded; however, Griffin (U.S. Pat. No. 5,469,927) has also disclosed a cutter with an array of star-shaped projections which extend into the cutting surface from the substrate.
Other prior art compacts have a cutting surface of superhard material which is thicker at the center of the cutter so that it projects into the substrate (Olmstead, U.S. Pat. No. 5,472,376). Alternatively, the superhard material may be thickest about the circumference of the compact (Flood et al., U.S. Pat. No. 5,486,137), on opposite sides of the compact (Tibbitts, U.S. Pat. No. 5,435,403), or on one side only (Flood et al., U.S. Pat. No. 5,494,477). In the Flood and Tibbitts patents, the thickness of the superhard material increases linearly from the central region of the cutter to the outer edge. These modifications to the geometry of the superhard layer are intended to reduce residual stresses in the cutter and thus reduce wear. In addition, by increasing the thickness of the superhard layer at the circumference of the cutter, where the most wear occurs, the lifetime of the cutter is increased. Other approaches to increasing the strength of compacts are to use polycrystalline diamond in reinforcing rods (Tibbitts et al., U.S. Pat. No. 5,279,375) or as a cylindrical core (Bovenkerk, U.S. Pat. No. 4,268,276). According to these references, the use of polycrystalline diamond inside a cutting element serves to reduce residual stresses.
A further prior art method for making the cutting action of a standard diamond table more effective is to use a "scribing" action. This can be accomplished by including pointed cutting elements on a drill bit along with cylindrical cutters, so that the pointed cutters cut grooves or kerfs into the rock surface so that it can be more easily cut by the blunter cylindrical cutters. This approach is described by Weaver (U.S. Pat. No. 4,602,691). Another approach is to wire electric discharge machine a point (parallel to bit rotation) into a standard polycrystalline diamond cutter (PDC), thus combining the scribing and standard cutting action in a single cutter. However, this cutter design has no additional diamond to provide greater wear resistance to the point and, consequently, the point is worn down in the first few hours of drilling. A scribing effect has also been attributed to DBS's "claw" cutters, as described in Dennis (U.S. Pat. No. 4,784,023). The "claw" cutter addresses the wear problem by providing additional diamond, but the parallel cutting action provided by the small diamond-filled grooves is minimal at best.
Each of the above patents is hereby incorporated by reference in its entirety.